1. Field of the Invention
The present invention relates to a liquid feeder for feeding liquid, such as ink or varnish, to a sheet held on an impression cylinder so as to perform, for example, printing or coating on the sheet. More particularly, the invention relates to a liquid feeder for use in a screen printing unit of a screen printing machine for performing screen printing on sheets.
2. Description of the Related Art
A conventional screen printing machine employs a rotary screen. The rotary screen includes a hollow cylinder rotatably supported and formed as follows: a thin screen (made of, for example, stainless steel or nickel) in which small holes are etched in image patterns is formed into a cylindrical shape. In the interior of the hollow cylinder are provided an ink fountain fixedly supported by a frame, and a squeegee. The squeegee pushes out ink through the small holes of the hollow cylinder, thereby printing the image patterns on sheets. In view of its capability of thick-application printing in special ink or the like, rotary screen printing is employed for imparting high quality to, for example, appearance and tactile impression.
When printing is performed on sheets by use of such a rotary screen, the following problem is potentially involved. Since a sheet is held on an impression cylinder, which is in contact with the rotary screen, by means of a gripper, projection of the gripper from the outer circumferential surface of the impression cylinder may cause damage to the rotary screen.
In order to cope with the above problem, Japanese Kohyo (PCT) Patent Publication No. 2000-504643 discloses the following technique. A gripper and a gripper pad are disposed in the interior of a notch of an impression cylinder so as to prevent projection of the gripper and gripper pad from the outer circumferential surface of the impression cylinder. An openable and closable cover is provided for covering the notch. When a sheet is to be gripped or released, the cover is opened or closed synchronously with gripping or ungripping of the gripper, thereby allowing gripping or releasing of the sheet and preventing damage to the rotary screen, which could otherwise result from protrusion of the rotary screen into the notch of the impression cylinder or contact between the rotary screen and a projecting object (gripper).
In order to cope with the above problem, Japanese Patent Application Laid-Open (kokai) No. 2001-225445 discloses the following technique. In place of the gripper and the gripper pad, a suction head is provided in the interior of the notch of the impression cylinder. A portion of the notch other than that where the suction head is provided is covered with a cover. When a sheet is to be held, the suction head is activated to suction-hold the sheet. When a sheet is to be released, the suction head is deactivated to release the sheet. In this manner, a sheet is held or released, and there is prevented damage to the rotary screen, which could otherwise result from protrusion of the rotary screen into the notch of the impression cylinder or contact between the rotary screen and a projecting object.
However, the impression cylinder described in Japanese Kohyo (PCT) Patent Publication No. 2000-504643 is rotated while the cover that covers the entire notch undergoes opening and closing. Accordingly, when the notch is covered with the cover, high-speed rotation of the impression cylinder may cause vibration of the cover, resulting in a failure to align the cover with the outer circumferential surface of the impression cylinder. Therefore, application of the disclosed technique to high-speed printing is difficult. Also, the vibrating cover may come into contact with the hollow cylinder of the rotary screen, causing damage to the rotary screen.
In the impression cylinder described in Japanese Patent Application Laid-Open (kokai) No. 2001-225445, the suction mechanism for suction-holding a sheet becomes complex, resulting in increased cost. Also, when a sheet is transferred to the suction mechanism from an upstream cylinder, deformation (undulation) of the sheet in the sheet width direction may cause failure of the suction mechanism to suction-hold the sheet.
Occurrence of such a problem is not limited to the case where thick-application printing is performed on sheets in special ink or the like by use of the rotary screen. Such a problem may also arise in the case where liquid is fed from a liquid-feeding cylinder to a sheet held on the impression cylinder, as in the case of application of varnish to a sheet by use of the rotary screen.